Detect Source Drift with Propeller.

Source Drift happens when the sources are updated after profiling the binary
but before building the final optimized binary. If the source has changed since
the profiles were obtained, optimizing basic blocks might be sub-optimal. This
only applies to BasicBlockSection::List as it creates clusters of basic blocks
using basic block ids. Source drift can invalidate these groupings leading to
sub-optimal code generation with regards to performance.

PGO source drift for a particular function can be detected using function
metadata added in D95495.

When source drift is deected, disable basic block clusters by default
which can be re-enabled with  -mllvm option
bbsections-detect-source-drift=false.

Differential Revision: https://reviews.llvm.org/D95593

llvm/lib/CodeGen/BasicBlockSections.cpp

@@ -88,6 +88,12 @@ cl::opt<std::string> llvm::BBSectionsColdTextPrefix(
     cl::desc("The text prefix to use for cold basic block clusters"),
     cl::init(".text.split."), cl::Hidden);
 
+cl::opt<bool> BBSectionsDetectSourceDrift(
+    "bbsections-detect-source-drift",
+    cl::desc("This checks if there is a fdo instr. profile hash "
+             "mismatch for this function"),
+    cl::init(true), cl::Hidden);
+
 namespace {
 
 // This struct represents the cluster information for a machine basic block.
@@ -313,10 +319,42 @@ static bool avoidZeroOffsetLandingPad(MachineFunction &MF) {
   return true;
 }
 
+// This checks if the source of this function has drifted since this binary was
+// profiled previously.  For now, we are piggy backing on what PGO does to
+// detect this with instrumented profiles.  PGO emits an hash of the IR and
+// checks if the hash has changed.  Advanced basic block layout is usually done
+// on top of PGO optimized binaries and hence this check works well in practice.
+static bool hasInstrProfHashMismatch(MachineFunction &MF) {
+  if (!BBSectionsDetectSourceDrift)
+    return false;
+
+  const char MetadataName[] = "instr_prof_hash_mismatch";
+  auto *Existing = MF.getFunction().getMetadata(LLVMContext::MD_annotation);
+  if (Existing) {
+    MDTuple *Tuple = cast<MDTuple>(Existing);
+    for (auto &N : Tuple->operands())
+      if (cast<MDString>(N.get())->getString() == MetadataName)
+        return true;
+  }
+
+  return false;
+}
+
 bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
   auto BBSectionsType = MF.getTarget().getBBSectionsType();
   assert(BBSectionsType != BasicBlockSection::None &&
          "BB Sections not enabled!");
+
+  // Check for source drift.  If the source has changed since the profiles
+  // were obtained, optimizing basic blocks might be sub-optimal.
+  // This only applies to BasicBlockSection::List as it creates
+  // clusters of basic blocks using basic block ids. Source drift can
+  // invalidate these groupings leading to sub-optimal code generation with
+  // regards to performance.
+  if (BBSectionsType == BasicBlockSection::List &&
+      hasInstrProfHashMismatch(MF))
+    return true;
+
   // Renumber blocks before sorting them for basic block sections.  This is
   // useful during sorting, basic blocks in the same section will retain the
   // default order.  This renumbering should also be done for basic block

llvm/test/CodeGen/X86/basic-block-sections-source-drift.ll

@@ -0,0 +1,19 @@
+; RUN: echo "!foo" > %t.order.txt
+; RUN: llc < %s -mtriple=x86_64-pc-linux -basic-block-sections=%t.order.txt  | FileCheck --check-prefix=SOURCE-DRIFT %s
+; RUN: llc < %s -mtriple=x86_64-pc-linux -basic-block-sections=%t.order.txt -bbsections-detect-source-drift=false | FileCheck --check-prefix=HASH-CHECK-DISABLED %s
+
+define dso_local i32 @foo(i1 zeroext %0, i1 zeroext %1)  !annotation !1 {
+  br i1 %0, label %5, label %3
+
+3:                                                ; preds = %2
+  %4 = select i1 %1, i32 2, i32 0
+  ret i32 %4
+
+5:                                                ; preds = %2
+  ret i32 1
+}
+
+!1 = !{!"instr_prof_hash_mismatch"}
+
+; SOURCE-DRIFT-NOT: .section .text
+; HASH-CHECK-DISABLED: .section .text